The rapid increase in population and developed land in the Chicago metropolitan area has placed a heavy demand on water resources. Owing to legal restrictions and natural limitations on the availability of additional water from Lake Michigan and the region’s deep aquifer system, the most cost-effective option for future water development in the region is likely to be the shallow aquifer system. The shallow aquifers of the region are vulnerable to surface-derived contaminants, and the increase in developed land may be increasing the rate at which groundwater quality is being degraded. Historical shallow groundwater chloride (Cl-) concentrations from the Chicago metropolitan area have been evaluated for data quality and
temporal trends. Chloride concentrations are increasing in municipal wells in the outermost counties of the Chicago metropolitan area, with road salt runoff likely the largest source of contamination. In the vast majority of municipal wells in DuPage, Kane, McHenry, and Will Counties, Cl- concentrations have been increasing. More than half of the wells in these four counties have rate increases greater than 1 mg L-1 yr-1 and approximately 13% have increases greater than 4 mg L-1 yr-1. On the other hand, Cl- concentrations have not been increasing in most municipal wells in Cook and Lake Counties. Approximately 16% of the samples collected
from municipal wells in northeastern Illinois in the 1990s had Cl- concentrations greater than 100 mg L-1; median values were less than 10 mg L-1 prior to 1960, before extensive road salting.

This document is a product of the Illinois Groundwater Consortium, and has been made available by the Illinois State Water Survey and the University Library, University of Illinois at Urbana-Champaign. It is intended for research and educational use, and proper attribution is requested.